U.S. Pat. Nos. 3,218,953 and 3,747,504 describe laboratory fume hoods with a rear particle wall and adjustable baffle adjacent this wall for directing gases from various locations in the fume hood to an upper exhaust port. The baffles described in these patents include an upper baffle with an obtuse angular bend at it center and a lower flat panel baffle. The upper panel is pivotally mounted to the rear wall at an apex of its obtuse angle, and the lower flat panel baffle is pivoted at its bottom end. By pivotally adjusting the two panel baffle system described in these patents, the fume hood can accomodate different weight fumes, i.e. gases. Three separate settings of these baffles handle fumes that are (1) approximately equivalent to air weight, (2) heavier than air, and (3) lighter than air. A schematic illustration of the various settings of the baffles of the above two patents is submitted with the prior art material for background.
U.S. Pat. No. 3,217,630 describes a single panel baffle of obtuse angular design for use with counter top fume hoods. U.S. Pat. No. 2,627,220 illustrates in FIG. 4 a two panel vent spaced immediately above a triangularly shaped deflector. The bottom edge of such deflector is joined to a rear wall of the fume hood and has merely to deflect gas away from the rear wall rather than toward the rear wall.
All of the above two panel baffle systems have a vent above which is mounted a fan that forces the fumes through a conduit to an area outside the building. In a walk-in-hood, the exhaust vent might be 7 feet or more above the floor. The problem is less critical in a counter top fume hood where the vertical rear wall gas suction passage is approximately 3-4 feet in length. However, even in a counter top hood there is some loss of gas sweeping action because of the length of this vertical suction passage. It is important to sweep a very large volume of fumes from all areas of the hood with as small a blower as possible. The reason for this is that extremely large blowers in the exhaust would generate such a tremendous air velocity in the fume hood that experiments within the hood could be hampered. In addition, as the blower size increases, the noise of the motor to power it generally increases.